It's all about the pattern

Solved: usb audio headphones muted when pugged in

Don’t you hate it in the computer field where something that was working fine for ages suddenly stops working? So this is what happened to me this time:

All of a sudden, when I plugged in my nice new Sennheiser USB headphones (PC165 USB) I couldn’t hear the sound. To get the sound to play, I’d have to go to the Audio MIDI Setup utility and toggle the mute button in the audio output settings. It had been working fine for a month, just plug it in and any audio output would just switch over from the speaker to th headphones.
So I called AppleCare tech support who said this was a Sennheiser problem, and I sent e-mail to Sennheiser who of course pointed back at Apple.

Well it turns out that to fix the problem, all I had to do was check the “Thru” checkbox on the audio input settings (in the Audio MIDI Setup Application). With that box checked, it works!